1. Field of the Invention
The present invention relates to an image forming apparatus that adopts electrostatic recording, electrophotographic recording, and the like, and in particular relates to an image forming apparatus that includes a reverse unit for reversing a recording material which has passed through a fixing unit.
2. Description of the Related Art
A full color image forming apparatus with an intermediate transfer system is described below with reference to FIG. 10, as an example of an image forming apparatus relating to the present invention.
In recent years, with the progress of downsizing and sophistication of image forming apparatuses, an image forming apparatus having a structure described below has been developed (for example, see Japanese Patent Application Laid-Open No. 2004-151389 (p. 12, FIG. 1)).
As illustrated in FIG. 10, the image forming apparatus includes a plurality of image forming portions 110. The plurality of image forming portions 110 forms latent images on photosensitive drums 111 which are image bearing members by using light, magnetism, an electric charge, or the like, and visualizes the latent images to obtain visible images. An intermediate transfer member 130 to which the visible images are sequentially transferred from the each image forming portion to form a multicolor image is located above the image forming portions 110. A transfer means 136 transfers the multicolor image on the intermediate transfer member 130 to a recording material P, and a fixing device 140 fixes the multicolor image transferred to the recording material P, on the recording material P.
A feeding portion 120 for conveying the recording material P to a transfer portion Te, the manual feed tray portion 171 and the sheet feeding cassette portion 121 for supplying the recording material P to the feeding portion 120 are each disposed below the transfer means 136.
The intermediate transfer member 130 uses an intermediate transfer belt 131 which is a rotating endless belt extended between a plurality of rollers. An exposure device 106 is located close to and below the plurality of image forming portions 110. The visible images formed on the photosensitive drums 111 by the exposure device 106 are primarily transferred onto the intermediate transfer member 130 by primary transfer charging devices 135. The primary transfer charging devices 135 are disposed so as to face the image forming portions 110 and the intermediate transfer member 130. The visible images primarily transferred from the plurality of photosensitive drums 111 are overlaid one on top of another on the intermediate transfer member 130, and the intermediate transfer member 130 rotates to convey the overlaid visible image to the position Te where the image is transferred to the recording material P. The visible image on the intermediate transfer member 130 is secondarily transferred onto the recording material P selected and fed from the manual feed tray portion 171 or the sheet feeding cassette portion 121, at the secondary transfer position Te by the secondary transfer roller 136. The secondarily transferred visible image is then fixed by the fixing device 140, as a result of which a full color image is obtained.
The image forming apparatus described above can be downsized significantly, because a paper passing path of the recording material P is short and each unit is compactly arranged. Moreover, a time period from sheet feeding to output can be shortened, which contributes to higher speed. Furthermore, the short sheet passing path reduces an occurrence frequency of a jam such as a sheet jam, so that a high-quality image forming apparatus can be realized.
A delivery roller pair 150, a delivery tray 160, and a duplex conveying path 170 are disposed downstream of the fixing device 140 in a conveying direction. The recording material P which has passed through the fixing device 140 is delivered to the delivery tray 160 via the delivery roller pair 150, in the case of simplex printing or after printing on the second side in the case of duplex printing. The following describes an operational procedure of the delivery roller pair 150 in the case where the recording material P which has passed through the fixing device 140 is the first side in duplex printing, with reference to FIGS. 11 and 12.
FIGS. 11 and 12 are schematic enlarged views of the fixing and delivery parts in FIG. 10. A delivery guide 145, a flapper 146, a delivery roller pair 150 made up of delivery rollers 151a and 151b, and a duplex conveying path 170 are provided downstream of a fixing nip FN in a conveying direction. The flapper 146 is rotatable, and is biased downward under its own weight. The delivery rollers 151a and 151b can rotate forward and backward.
A reverse movement of the recording material P after the printing on the first side in duplex printing ends is described below. In FIG. 11, having passed through the fixing nip FN, the recording material P is conveyed to the delivery roller pair 150 via the conveying guide 145. The delivery roller pair 150 rotates in a direction of an arrow E, to convey the recording material P which has passed through the fixing nip FN until a rear end of the recording material P completely comes out of the fixing nip FN. The recording material P is conveyed while pushing up the flapper 146 which is placed in the conveying guide 145 and biased in a gravitational direction under its own weight.
After the rear end P2 of the recording material P completely passes through the flapper 146, the delivery rollers 151a and 151b rotate backward in a direction of an arrow F as illustrated in FIG. 12, to convey the recording material P to the duplex conveying guide 170. Having been conveyed to the duplex conveying guide 170, the recording material P is conveyed to the transfer unit again, in order to print on the second side in duplex printing.
Such a structure in which the delivery roller pair is disposed immediately downstream of the fixing unit and the sheet conveying direction is changed between simplex printing and duplex printing by the forward and backward rotation of the delivery roller pair contributes to component simplification and apparatus downsizing.
However, the following problem arises in the image forming apparatus shown in the above-mentioned conventional example. In the case of continuously printing a plurality of recording materials in duplex printing, while the delivery roller pair is rotating backward to convey one recording material to the duplex conveying guide, the next recording material cannot advance into the delivery roller pair. Accordingly, an interval (hereafter referred to as a sheet interval) between one recording material and the next recording material in continuous sheet passing needs to be approximately as long as a result of subtracting a distance between the fixing unit and the delivery unit from a length of the recording material in a sheet passing direction. This causes a significant decrease in productivity.
In view of the above-mentioned problem, Japanese Patent Application Laid-Open No. 2008-008950 (p. 9, FIG. 1) proposes a structure of separately providing a delivery roller pair and a reverse roller pair. According to the structure of this patent document, even when continuously printing a plurality of recording materials in duplex printing, delivery and reversal can be performed approximately at the same time. This enables the sheet interval to be minimized, thereby enhancing the productivity.
However, in the proposed structure of Japanese Patent Application Laid-Open No. 2008-008950 (p. 9, FIG. 1), the fixing unit and the reverse roller pair are installed in the image forming apparatus independently of each other. This causes deterioration in alignment of the fixing unit and the reverse roller pair, depending on precision of a plurality of components. When the alignment of the fixing unit and the reverse roller pair deteriorates, in a state where the recording material is sandwiched by the fixing unit and the reverse rollers, the recording material is caused to form a deviated loop between the fixing unit and the reverse roller pair. Besides, an excessively large deviated loop may induce a sheet jam in the conveying guide. Even if the deviated loop is not so large as to induce a sheet jam, when the rear end of the recording material passes through the fixing unit and is reversed by the reverse roller pair, the amount of deviated loop directly becomes the amount of skew feeding of the recording material. The recording material is conveyed to the duplex conveying guide in a skew feeding state and the second side in duplex printing is printed, as a result of which an image of the second side is printed askew on the recording material.
For precise alignment of the fixing unit and the reverse roller pair, it is necessary to increase the precision of each component. This makes component management in mass production difficult, and also leads to an increase in cost. The effect on the alignment precision of the fixing unit and the reverse roller pair can be reduced by providing a structure of correcting the skew fed recording material before printing on the second side. However, this requires a new component to be added, which hinders apparatus downsizing and also causes a cost increase due to an increased number of components.